Recently, cooperative communications have been studied to exploit diversity in order to achieve better network performance. The concepts for CoMP are proposed by 3rd Generation Partnership Project (3GPP), e.g. in TR 36.814, release 9, “Evolved Universal Terrestrial Radio Access (E-UTRA); Further advancements for E-UTRA physical layer aspects”. CoMP refers to a system where the transmissions and/or reception at multiple, geographically separated antenna sites are coordinated in order to improve system performance. CoMP may significantly enhance cell average and cell edge throughput. However, the transmissions must be carefully coordinated to avoid causing undesired interferences.
There are three typical types of DL CoMP coordination mode discussed by 3GPP. The first mode is Coordinated Scheduling (CS), as shown in FIG. 1A. In the CS, each UE is served by only one site. The site hereinafter may be a macro/micro/pico cell, evolved NodeB (eNB), a relay cell, a repeater, etc. For example, a UE 101 is served by an eNB 111, and a UE 102 is served by an eNB 112. There are certain coordinations between the two eNBs, as indicated by dashed lines. The CS mode is to schedule the DL transmission from the eNB 112 to UE 102 in different time-frequency resources than the transmission from the eNB 111 to UE 101, and vice versa, so that the experienced interference from the eNB 112 to the UE 101 is reduced.
The second mode is Coordinated Beamforming (CB), as shown in FIG. 1B. In this mode, the interference caused by DL transmissions from the eNB 112 to the UE 101 can be reduced by spatially nulling the beams targeting the UE 101 (as shown in dashed arrows) by certain coordination of the precoding between the eNBs. The interference caused by DL transmissions from the eNB 111 to the UE 102 can be also reduced by spatially nulling the beams targeting the UE 102.
The third mode is coherent Joint Transmission (JT), as shown in FIG. 1C. In this mode, multiple sites transmit the same signals at least over a subframe, to the same UE simultaneously using the same resources. For example, the eNBs 111 and 112 transmit the same signals to a UE 103 using the same resource. The signals from the two eNBs are coherently combined in the air interface when they reach the UE 103, and the signal to noise and interference ratio (SINR) at the UE 103 may be improved.
The coherent JT is transparent to the UE if UE specific DeModulation Reference Signal (DM-RS) is utilized and coherently transmitted by the serving sites of the UE. However, each serving site for the UE is required to allocate the same full set of resource blocks to transmit the same subframe to the UE. In other words, if one site is to transmit a subframe of signal to a UE using a set of resource blocks, then another site can not join the coherent JT unless it has the same set of resource blocks available to transmit the same subframe. Such requirement limits the time frequency resource utilization and restricts the scheduling and link adaptation.